The Serie A leaders have an option to buy the England international for a fee around €30 million in the summer.
The 23-year-old broke into the Chelsea senior team last season after a successful loan spell at Derby County in the 2018/2019 season where he was voted as the Rams’ player of the year.
Tomori was a regular starter in Frank Lampard's side at that point and won his only full England cap in November 2019 in a game against Kosovo.
The former Hull City and Brighton loanee signed a five-year deal with Chelsea in December 2019.
However, since the arrival of veteran defender Thiago Silva, Tomori has found games hard to come by, only making four appearances for Chelsea in all competitions.
Chelsea and AC Milan took to their various official websites to announce Tomori’s deal on Friday.
“AC Milan are delighted to announce the signing of Oluwafikayomi Oluwadamilola Tomori on loan from Chelsea FC until the end of the season,” Milan wrote in a statement.
“The defender has signed a contract until June 30, 2021, but AC Milan have an option to make the deal permanent.”
“Fikayo Tomori will spend the rest of this season on loan with Serie A leaders AC Milan,” Chelsea wrote on their official website.
"The Academy graduate will continue his development at the San Siro having been with Chelsea since Under-8s level. He has made 27 appearances for the Blues so far, including four this season, most recently as a substitute in the FA Cup third round win over Morecambe earlier this month.”
